Shooting for PARs in lung diseases

Abstract

Proteinase-activated receptors (PARs) are novel G-protein-coupled receptors activated by serine and other proteinases to induce changes in cellular function. There is extensive evidence that PARs are expressed in the airways in a variety of cell types that are relevant to inflammatory lung diseases, and that activation of these receptors might be linked to significant pathological changes. Thus, PARs are exciting new targets in lung disease research. However, much of the data to date has come from in vitro studies using limited pharmacological tools, and considerably more needs to be known about the functions of this family of receptors in the lung before their potential as drug targets can be established.

Introduction

Proteinase-activated receptors (PARs) are, on the face of it, similar to many other cellular receptors. They are seven transmembrane-spanning G-protein-coupled receptors, and their activation evokes key changes in the cell, such as alterations in free intracellular calcium and proliferation. The unique feature of these receptors is that they bear their own peptide ligand hidden within the extracellular N-terminal sequence; a ligand that is revealed only when the action of a proteinase removes the existing N-terminal sequence, which otherwise prevents it from binding (Figure 1). Once this blocking sequence is removed, the new N-terminal sequence (or tethered ligand) can interact with the second transmembrane domain of the receptor to activate cell signalling events. The first receptor to be cloned, now designated PAR1, was found at the end of a long investigation into the cellular actions of the coagulation proteinase thrombin. Subsequently, three other PARs have been cloned in humans and mice (Table 1). There are several excellent reviews of the general properties of PARs 1., 2., 3., and the International Union of Pharmacology has published nomenclature guidelines [4^•] to which we adhere in this review.

All the PAR genes share a similar two exon structure [5]. PAR₁, PAR₂ and PAR₃ are located close together on a single chromosome in humans (5q13) and mice (13D2), whereas the gene for PAR₄ is located separately in both species (chromosome 13D2 and 8B3.3, respectively). Similarly, multiple alignment of the amino acid sequences of PARs indicates that PAR₄ stands out as the most unique receptor (Figure 2). The similarity between murine and human PAR proteins indicates that they are genuine homologues, supporting the current nomenclature. Also, the existence of an identifiable PAR₁ homologue in amphibians indicates just how ancient PARs are as a receptor ‘concept’.

The similar structure of the genes for PAR₁, PAR₂ and PAR₃, and their proximity to one another, suggests that they may have evolved by gene duplication events from a single ancestral gene. Therefore, the enigmatic PAR₂ appears to have arisen from thrombin receptor ancestry, with subsequent modification of the cleavage site to confer specificity for trypsin. When PAR₄ was cloned, it was thought that a search near the novel location of the PAR₄ gene might yield another locus of similar receptors. However, no new PAR has yet been cloned.

The clearest physiological role for PARs is on platelets, and it is here that a most significant species difference in receptor utilisation occurs. Human platelets rely upon a combination of PAR₁ and PAR₄ for responsiveness to thrombin, with PAR₁ acting as the higher affinity receptor. Conversely, murine platelets express PAR₃ and PAR₄. Murine PAR₃ does not appear to be linked to any signal transduction pathways, yet it is important in clot formation as it acts as a high affinity site for thombin binding, which is then thought to facilitate subsequent cleavage of PAR₄. By contrast, human PAR₃ has been shown to signal to thrombin when expressed in cells, but its physiological roles remain obscure. Thus, different species use different PARs to achieve the same end — something that should be borne in mind as a potential problem when studying animal models of human diseases.